Logging techniques for determining numerous borehole and formation characteristics are well known in oil drilling and production applications. Such logging techniques include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, acoustic velocity, acoustic caliper, downhole pressure, and the like. In conventional wireline logging applications, a probe having various sensors is lowered into a borehole after the drill string and bottom hole assembly (BHA) have been removed. Various parameters of the borehole and formation are measured and correlated with the longitudinal position of the probe as it is pulled uphole. More recently, the development of logging while drilling (LWD) applications has enabled the measurement of such borehole and formation parameters to be conducted during the drilling process. The measurement of borehole and formation properties during drilling has been shown to improve the timeliness and quality of the measurement data and to often increase the efficiency of drilling operations.
LWD tools are often used to measure physical properties of the formations through which a borehole traverses. Formations having recoverable hydrocarbons typically include certain well-known physical properties, for example, resistivity, porosity (density), and acoustic velocity values in a certain range. Such LWD measurements may be used, for example, in making steering decisions for subsequent drilling of the borehole. For example, an essentially horizontal section of a borehole may be routed through a thin oil bearing layer (sometimes referred to in the art as a payzone). Due to the dips and faults that may occur in the various layers that make up the strata, the drill bit may sporadically exit the oil-bearing layer and enter nonproductive zones during drilling. In attempting to steer the drill bit back into the oil-bearing layer (or to prevent the drill bit from exiting the oil-bearing layer), an operator typically needs to know in which direction to turn the drill bit (e.g., up, down, left, or right). In order to make correct steering decisions, information about the strata, such as the dip and strike angles of the boundaries of the oil-bearing layer is generally required. Such information may possibly be obtained from azimuthally sensitive measurements of the formation properties and, in particular, from images derived from such azimuthally sensitive measurements.
Downhole imaging tools are conventional in wireline applications. Such wireline tools typically create images by sending large quantities of azimuthally sensitive logging data uphole via a high-speed data link (e.g., a cable). Further, such wireline tools are typically stabilized and centralized in the borehole and include multiple (often times one hundred or more) sensors (e.g., resistivity electrodes) extending outward from the tool into contact (or near contact) with the borehole wall. It will be appreciated by those of ordinary skill in the art that such wireline arrangements are not suitable for typical LWD applications. For example, communication bandwidth with the surface is typically insufficient during LWD operations to carry large amounts of image-related data (e.g., via known mud pulse telemetry or other conventional techniques).
Several LWD imaging tools and methods have been disclosed in the prior art. Most make use of the rotation (turning) of the BHA (and therefore the LWD sensors) during drilling of the borehole. For example, U.S. Pat. No. 5,473,158 to Holenka et al. discloses a method in which sensor data (e.g., neutron count rate) is grouped by quadrant about the circumference of the borehole. Likewise, U.S. Pat. No. 6,307,199 to Edwards et al., U.S. Pat. No. 6,584,837 to Kurkoski, and U.S. Pat. No. 6,619,395 to Spros disclose similar binning methods. In an alternative approach, U.S. Pat. No. 7,027,926 to Haugland, which is commonly assigned with the present invention, discloses a method in which azimuthally sensitive sensor data are convolved with a predetermined window function. Such an approach tends to advantageously reduce image noise as compared to the above described binning techniques.
LWD data are conventionally transmitted uphole (to the surface) via mud pulse telemetry techniques. Such techniques are typically limited to data transmission rates (bandwidth) on the order of only a few bits per second. Since LWD imaging sensors typically generate data at much higher rates than is possible to transmit to the surface, borehole images are often processed from data stored in memory only after the tools have been removed from the wellbore. Significant data compression is required to transmit images to the surface during drilling. While the above described binning and windowing techniques do provide for significant data reduction, significant further data compression is necessary in order to transmit images to the surface in a timely fashion (e.g., such that the borehole images may be utilized in steering decisions). Mud pulse telemetry techniques also tend to be error prone. Thus, a suitable LWD image compression scheme requires a high degree of error resilience. Furthermore, payzone steering (with LWD) is highly sensitive to latency as a delayed response allows the drill bit to potentially continue drilling in the wrong direction. Hence, low latency compression and transmission is highly desirable.
Transform coding techniques are known in the art. For example, U.S. Pat. No. 6,405,136 to Li et al. discloses a method for compressing borehole image data, which includes generating a two-dimensional Fourier Transform of a frame of data, transmitting a quantized representation of some of the Fourier coefficients to the surface, and applying a forward Fourier Transform to the coefficients to recover an approximate image at the surface. The use of discrete cosine transforms (DCT) and wavelet transforms are also known in the art. One drawback with the Li et al approach is that relatively large, two-dimensional data frames (16×56) are required in order to get sufficient compression, which thereby increases data latency (the time delay between when the data is generated downhole and received at the surface).
Therefore there exists a need for an improved data compression method, and in particular a data compression method suitable for sufficiently compressing LWD image data so that the compressed data may be transmitted to the surface via conventional telemetry techniques.